The present invention relates to a variable speed friction disc transmission having a pair of discs with a friction or driven disc having its periphery engaging the face of a drive disc. Friction disc transmissions of the type disclosed herein are used primarily of propulsion of small land traversing machines, particularly power lawn mowers.
A variable speed friction disc transmission of the type disclosed herein is shown in U.S. Pat. Nos. 4,169,392 and 4,173,153, assigned to the assignee of the present invention. The transmission disclosed in the aforementioned patents includes a driving disc rotatable about a vertical axis having a flat horizontal force normal to its axis of rotation and a driven disc rotatable on an axis normal to the axis of the driving disc and having its periphery engaging the face of the driving disc. The driving disc is rotatably supported on a swing arm which is swingable about a vertical axis.
The speed at which rotation is imparted to the driven disc is a function of the distance between the axis of the driving disc and the zone at which its face is engaged by the periphery of the driven disc. Further, the driven disc rotates in one direction on one side of the rotational axis of the driving disc and in the opposite direction on the opposed side.
The friction disc transmission disclosed in U.S. Pat. Nos. 4,169,392 and 4,173,153 has the disadvantage of causing rapid acceleration starts when the driven disc contacts the driving disc upon release of the clutch pedal. The rapid acceleration results because of a self-energizing feature that exists in the construction which causes the contact pressure between the driven and driving discs to be proportional to the torsional load on the wheel axle. The initial engagement between the driven and driving discs produces slippage which is sensed and abruptly corrected resulting in rapid acceleration.
Thus, there has been a need for a friction disc transmission which provides a smooth start when the friction discs are initially engaged while being self-energizing to overcome slippage under load.